Roughness measuring means



B. A. COSS ROUGHNESS MEASURING MEANS April 9, 1946.

Filed Feb. 2e, 1943 2 Sheets-Sheet l April 9, 1946. B` A COSS ROUGHNESSMEASURING MEANS Filed Feb. 26, 1943 2 Sheets-Sheet 2 INVENTOR. BY BERT/COsS Patented Apr. 9, 1946 2,397,923 ROUGHNESS MEASURING MEANS Bert A.Cess, St. Louis Park, Minn., assigner to The Brush Development Company,Cleveland, Ohio, a corporation of Ohio Application February 28, 1943,Serial` No. 477,242

(CL I3-105) Claims.

My invention pertains to improved apparatus for measuring roughness ofsurfaces; and more particularly to apparatus for giving the percentageof a surface which has a roughness which exceeds a certain set limit.

One of the knownfmethods of attacking the problem of surface roughnessmeasuring is to produce an alternating electric current corresponding tothe minute irregularities and there` after to make, automatically, avisually observable record corresponding thereto, such as. a graph.Apparatus for that purpose may comprise a device analogous' toy aVphonograph pickup of the hill-and-dale type having a stylus that isresiliently heldin contact with the surface undergoing test, andsimultaneously is moved transversely thereof. The pickup may be of theelectromagnetic'type that gives a varying output voltage proportional tothe velocity of movement of the stylus in the vertical direction as itfollows the surface irregularities, exemplified by the United Statespatent to Abbott 2,240,278, or it may be of the typeincludins apiezoelectric crystal element. A device of the latter type is em ployedin thesurface Analyzer manufactured and sold by The Brush DevelopmentCompany of Cleveland, Ohio; it providesl an output' potentialproportional to the excursions of the tracing stylus when the pickup ismoved over a surface, which output, after amplification, may be utilizedto actuate a pen recorder, or, if a permanent record is not desired, tocontrol the denection of they beam in a cathode ray oscilloscope, or tocontrol a meter.

'Roughness measuring devices of the general type referred to have givenexcellent results in service; they do have certain limitations. however,which are overcome by the present invention. For example, when measuringeither the average or the root-mean-square roughnessofy a surface,usually expressed in microinches, dimculty sometimes is experiencedbecause the surface, if extended, may vary in its average roughness frompoint to point. Accordingly, a meter which is intended to indicateeither the average or the root-mean-square of the displacement of astylus that scans such a surface vunder test will give a continuouslyvarying reading. The reading, to say the least. may be somewhatambiguous and the ambiguity may increase in proportion to the extent oflongitudinal travel of the stylus.

v It is, accordingly, an object of my invention to provide an apparatusfor roughness measuring that shall give a single definite indicationdevoid of ambiguity.

Another object of my invention is to provide apparatus for roughnessmeasuring that will give an unequivocal indication of the amount ofsur-l face having a roughness which exceeds a fixed amount.

An object of my invention is to provide means for measuring a new valueof surface quality.

A further object of my invention is to provide a circuit for use with asurface measuring device' which will read the percentage of time thatthe ridges or scratches in a surface exceed a set tolerance limit.

Itis also an object ofl my invention to provide a surface measuringdevice which will readthe' percentage of time that the ridges orscratches in al surface exceed a. set tolerance limit, andy to have thevalue of the tolerance selectively adjustable. l

Anotherl object of myl invention is torbe'; able'torv count the peakswhichl exceed a. certain fixed amplitude limit.

Still another object of my invention is tofprov vide the means fordetermining the amplitude distribution` of aL rough surface'. i

Other objects and-a fuller understanding ofl my invention will behestunderstood from the following description and claims, when read in con"-nectionk with the accompanying drawings, in'

which:

Figure v1 is a conventionalized view, partly diagrammatic and partly inlcross-section, showin!- one form of my invention.

Figures 2 to 'l inclusive show the effect of the circuit on a wave-formof a sinusoidal signalat different points inthe circuit.

'Figui-e 8 is a graph illustrating a wave-form' and the progressivedeflection caused by it on an indicating meter calibrated to read thepercentageof time the roughness of` a surface exceeds a cer`'v tainfixed value.

Figure 9 shows a rectiedslnusoidal wave'. Figure 10 is a circuit diagramof a portionv of the circuit of my device, showing the details ofV thebuffer amplifier and the limiter.

In Figure 1 the specimen whose surface is to bel measured is indicatedby reference character I5,

and is held in a holder I6 which rests on a base I1. 'I'he device formeasuring the surface is in-y dicated generally by reference characterIt, and f comprises a motor` drive system for reciprocally actuating thearms I 9 and 22 to .move thetwo styli 20, 2i across the surface ofspecimen il The stylus 2li is large to prevent it from followingv thesmall surface scratches and to enable it to" bear the weight of the' arm22 which is pivotally connected to arm I9. The stylus' 2| has a sharppoint which enables it to follow the small scratches, and it isconnected to a piemelectric crystal 23, which may be of the Rochellesalt type, and which is connected to the arm 22. For large surfacewaviness both styli 20, 2l are actuated, thereby causing arm 22 to pivotwith respect to arm I9. There will be little or no voltage outputfromthe piezoelectric crystal because there will be substantially norelative motion between the two styli. When the specimen has surfaceroughness 'as well as surface waviness there will be a diiferentialmotion between the' two styll 29, 2i

which will stress the crystal 23 and result in a voltage output betweenlines- 24, 25 bearing a relationship to the amount of the dinerentialmowhich has a push-pull output. through a fullwave rectifier, and thenthrough a buffer ampli fier 21 to a limiter 28, and then to a vacuumtube voltmeter 29.

l to connect the output of the calibrating signal supply 33 to the inputof the ampliiier 26, and

by adjusting the potentiometer 35 so that there is no bias on the tubes33, 34. The amplitude of the alternating signal from the supply 39should v tion. The voltage output is fed to an ampliiler 25 The outputcircuit of the amplier includes two rectifier tubes 33, 34, connected ina full wave rectiier circuit and developing rectiiled signal across theresistor 31. In series with the load resistors! in a bias battery 35 anda bias adjusting potentiometer 36. The battery 35 is poled to oppose theiiow of current through the rectiiers 33, 34, and as a result currentiiows only when the signal peaks exceed the bias. The

amount of bias can be manually regulated by i battery and thepotentiometer 35. The limiter serves to chop oil and throw away thepositive and negative peaks of the cycles, and the value at which thelimiting action commences should ybe just slightly greater than thevalue of the bias on the full wave rectifier. The output of the limiterthus consists of a series of unidirectional current pulses of equalamplitude but' of varying duration related to the length of time forwhich the corresponding peaks exceed the limiting value. Theseunidirectional pulses charge condenser 49 through resistor 44. Theresistor 44 should be so large that the condenser 49 is able to chargeup to only a. small fraction of the voltage of the pulsating output ofthe limiter 25 Aduring one stroke of the 'pickup over the speci- ,tubevoltmeter 29 indicates the voltage across the condenser 49. Preferablythe output impedance of the limiter shouldV be extremely high-dur-v ingintervals between pulses so that condenser 49.will not tend to dischargeback through the limiter. vThe circuit should be so designed that whenthev potentiometer 35 is set for zero bias a full trace of the stylussystem across the speci-` Aset limit.

be great enough to cause limiting action substantially completely overeach half-cycle, and due to the timer 45 the signal is applied to themeter for a period of time corresponding to the length of time for onetrace of the stylus across the,l

specimen. rThe vacuum tube voltmeter will then give a reading whichshould be indicating that a. signal is being applied to it substantiallyall of the time. If the meter does not read 100% the condenser 43 shouldbe adjusted and the calibrating process repeated until the properreading is obtained. Means other than using a variablecondenser 49 couldbeused, such, for example. as means for adjusting the voltmeter 29.

The calibration of the second component of the circuit is based on thefact that the output of the pickup system for a given stylus displace--ment is known, and on the fact that for a sine wave of known amplitudeit is possible to compute the percentage of time that the wave exceedsany given level, and therefore it is possible to calibrate thepotentiometer 35 which is used to set the given level. I introduce fromthe calibratingsignal supply 39 into the circuit to be calibrated a sinewave having an amplitude related by a known factor to the amplitude ofthe pickup output for a given stylus displacement. Then ythepotentiometer is set for a convenient known reference level, and theamount of current from battery 35 is adjusted by means of the variableresistor 54 until the meter 29 (which has already. been calibrated)-reads the correct percentage of time that the wave exceeds the Theprinciple of the calibration method andthe procedure to follow may bestbe illustrated by a numerical example which will lalso indicate somefactors to be considered in designingl a surface measuring device ofthis type.

' Assume that it may be desirable to set the ref-- erence level by meansof the potentiometer 35 at any value up to 20 micro-inches of roughness.'I'he potentiometer 35 would then be provided with alinear scale readingfrom zero to 20 microinches; each single division representing aresistor value corresponding to one micro-inch of surface roughness.Also assume from the manu'- facturers specifications relating to theypiel-:up system that it is known that the pickup delivers 1.2millivolts per micro-inch of stylus displacement. Then the sinusoidaloutput signal from the calibrating signal source 39 is adjusted bymeansof the potentiometer 4l to have a peak value conveniently related to theoutput of the pickup, say of 12 millivolts, which corresponds to a.stylus displacement ofthe pickup system of 10 micro-inches. Set the biaspotentiometer at come predetermined convenient value, say, for

instance. a resistor value corresponding to 8.66

asaaoas micro-inches. It is easy to determine that for a sine wavehaving an output of units, the wave exceeds a level of 8.66 unitsone-third of the time. This relationship is illustrated in Figure 9 inwhich the amplitude of the peaks of the rectiled sinusoidal signal havea value of 10 units and the bias has a value of 8.66 units. The time tduring which the signal exceeds the bias can be computed as 331/3percent of the time. Therefore, adjust the amount of current flowingfrom battery 35 through the potentiometer 36 by means of the variableresistor 54 until one trace of the pickup causes the meter 29 to read33.3 percent.

After Calibrating both components of the circuit, the switch 38 isthrown to connect the output of the crystal 23 to the amplifier 29, andthe potentiometer 39 is adjusted to set any desired bias value. Thevalue of the bias set by the potentiometer 39 and the roughness qualityof the surface of the specimen I9 determine the reading on the meter 29.For instance, a specilication might read that not more than 10 percentof a surface have roughness which exceeds 20 micro-inches. Thepotentiometer 39 is then set to establish a bias which corresponds to 20microinches, and the motor in the surface measuring device I8 is startedto cause the styli 20, 2| to move across the surface of the specimen.All roughness encountered by the stylus 2| which is less than 20.micro-inches will fail to establish a voltage of suilicient magnitude topass current through the rectifiers 33, 94, but each portion of thesurface which has roughness exceeding 20 micro-inches will establish avoltage which causes either tube 33 or tube 34 to pass a current thatproduces an indication on lthe meter 29. The meter 29 gives a reading ofthe percentage of time during a Acomplete stroke or trace of the stylussystem across the specimen that the roughness is greater than the setlimit of 20 microinches. The meter 29 is calibrated and all readingsshould be taken with a complete stroke or trace of the stylus across thespecimen. Any

' length of trace may be used but we prefer to utilize one-half cycle ofthe reciprocating arm I9. The length of trace on usual devices is on theorder of a fraction of an inch, but it is obvious that any other lengthof trace, either larger or shorter may be utilized. It is also obviousthat if the meter 29 is calibratedwith respectto-a complete trace of thestylus system across the specimen, and during the testing of a specimenl the meter 29 is read at some portion of the trace other than thecomplete trace, then the reading for a complete trace can be computed.For example: if the meter 29 is read at av time when the 'stylus systemhas' completed one-haii.' of the trace, then a substantially truereading can be found by multiplying the meter reading by two.

lA timing device 45 is provided forregulating the length of time theoutput signal from the crystal 23 is applied to the meter 29. The timer45 may be an accurate clock device. an electronic timer, or a mechanicalswitch. As shown, it is mechanically or electrically connected by line49 to the reciprocating arms I9, 22 and is mechan ically or electricallyconnected by line 41 to a lswitch 49 across the input circuit to themeter 29. The connections are such that when the stylus system startsits trace across the specimen the circuit from the crystal 23 to themeter is made through the timer 49, and the switch 49 is open; and atthe end of the trace the circuit from the crystal 23 to the meter 29 isbroken. After a time interval sumcient for an operator to read the meter29 and preferably during the retu'n stroke of the reciprocating arms I9,22, the timer automatically closes switch 48 to discharge theo condenser49 and reduce the meter reading to zero preparatory to the next reading.It is not essential that the timer automatically close `the switch 48 asthe operator could easily do it by- 9; and Figure 4 illustrates theappearance of the f signal across the other output circuit of the push-fv pli amplifier 29 prior to rectication such as at point 4. Figure 5illustrates the shape of the wave at point 5 in the circuit showing theeffect n on the sisnal of full wave rectification and the bias suppliedby the battery 3l. Figure 6 shows rectiiied signal after furtheramplification such as at point 9 in the circuit, and Figure 'I illus-jtrates the eifect of the limiter 28 on the signal as it appears acrossthe input to the vacuum tubey voltmeter 29, such as at point 1. Eachsuccessive e pulse charges the condenser 49 to an extentdepending uponits length, and the vacuum tube4 voltmeter 29 reads the voltage acrossthe con-l denser 49 corresponding to the Baccumulated charges producedby the successive pulses, thereby giving a reading which is anindication of the percentage of the surface having a roughness exceedinga value set by the bias.

Figure 8 illustrates graphically how the charge is built up on thecondenser 49 during a traceV across a surface which is very rough.Starting at zero percent as the stylus starts its trace, the chargeincreases vdue to each signal which ex ceeds the set bias. Eachhorizontal portion of lthe curve represents periods when peaks fallshort of the limiting value established by thel bias battery 99 andpotentiometer 99; and each rising portion ofthe curve represents periodswhen the condenser is being charged by pulses gorresponding to peakswhich exceed the limiting las.

If it is desired to obtain a reading of thepercentage o! the surfacewhich has peaks or which has scratches exceeding a certainlimit,separate bias controls, one for each rectifier tube 99, 34 should beused instead of the one shown, in order to provide independentlyadjustable reference levels for peaks and scratches. For obtaining areading of the percentage of the surface which has scratches exceeding acertain limit, the limit would be set by the bias value of thepotentiometer to obtain the desired limit, and the switch I9 opened.Signals corresponding to ridges in the specimen will not pass to themeter 29 lbut signals corresponding toscratches in the specimen will,and the meter 29 will therefore indicate the percentage of the surfaceof the specimen which has scratches exceeding the set limit. If it isdesired to obtain a reading of the percentage oi the surface which hasvridges exceeding a certain limit the bias value is set and a tracemadewith switch 99 closed and switch 9| opened. Obviously for these types ofreadings the device must be recalibrated.

To determine the number of peaks of the surface which exceed a certainadjustable limit the output from my circuit may be integrated and putthrough a half-wave rectiner. The output is then applied tothe meterwhich gives a reading proportional lto the number of peaks which*exceedl the set limit. By running a number of `traces with a series ofdifferent bias-values the amplitude distribution of the suriaceroughness can be determined.

In rsome yapplications it may be undesirable t :employ the timer l andthe meter reset switch Il. Amodiilcation ofthe circuit will permit thisalthough the `readings are apt to be somewhat less reliable, but yetcommercially usable. One way of modifying the circuit is to omit thetimer ment the limiter supplies pulses which tend to charge thecondenser Il and the leak tends to discharge the condenser. The resultis an "average charge on the condenser depending on the percentage oftime that pulses flow (caused vby peaks exceeding the bias value). Whenthe wave form o'f the voltage applied to the amplifier 25 is "regularfasis the case when the sine wave calibrating voltage is applied. the meterwill assume. aftersome short time delay, a steady reading which .will becorrect if the previously described calibrating procedure is followed(except meter has reached a steady value).

, that the length of time of applying the test signal need notbecontrolled beyond insuring that the For irregular wave forms such as areencountered in measurlng the usual surface, the meter reading williiuctuate somewhat like the action of the meter t in present average orRMS reading instruments.

Another modiilcation would be to replace the f vacuum tube voltmeter 2l,condenser 49. leak, and

resistor 4I by an A. C. voltmeter having consider- A' able inertia anddamping.

` Although I have described my invention with a ,certain degree ofparticulares', u is to be undert stood that the present disclosure hasbeen made only by way of example and that numerous changes in thedetails of construction and the combination and arrangement of parts maybe re- `sorted to without departing from the spirit and the scope o'fthe invention as hereinafter claimed.

v'I claimed as my invention:

quantity related to excursions of said point with respect to the carriermeans, means for amplifying the electrical quantity. means for limitingthe "peaks of the electrical quantity at a certain value,

means for producing a signal constant in amplitude and varying induration in accordance with the i lengths of time the peaks of theuctuating electrical quantity are limited, means for integrating saidsignal with respect -to time,.and means for measuring the integratedquantity to provide an indication of the percentage of a surface whichand switch and to` connect a leak resistance in parallel with condenserIl. With this arrangerelativev movement, means i'orI amplifying saidelectrical quantity, means for rectii'ying said electrlcal quantity,means for limiting the peaks-of the -electrical quantity, means forestablishing a signal constant in amplitude and varying in duration inaccordance with the time the peaks of the fluctuating electricalquantity are limited, means for integrating the constant signal withrespect to time, and means for measuring the integrated output. l

4. A device as set forth in claim 3 in which said rectifying meansincludes bias means.

5. A device as set forth in claim 3 in which said rectifying means is ahalf-wave rectifier.

6. In combination, an electrical pickup device of the type includingcarrier means on whichare mounted a movable stylus and means operable bysaid stylus for establishing a fluctuating electrlcal quantity relatedto excursions of said stylus with respect to the carrier means, meansfor amplifying said electrical quantity and having a push-pull output, afull-wave rectier connected to the output of saidampliiier, bias meansconnected to said rectifier for passing only the'peaks of saidfluctuating signal, limiter 'means connected to the output ofsaid'biased rectifier for limiting the peaks of said fluctuating signal,integrating means for integrating said biased and limited signal withrespect to time, and means for measuring said integrated signal.

7. A device as set forth in claim 6 in which said bias means areadjustable.

8. A device as set forth inclaim 6 in which the limit set by saidlimiter is adjustable.

9. A device as set forth in claim 6 in which said bias means areadjustable, and in winch the limit set by said limiter is adjustable andis set slightly higher than the value set by the bias means.

10. In combination, an electrical pickup device of the type includingcarrier means on which vare mounted a movable stylus andv means operableby said stylus for establishing a iluctuating elecexceeds a certainroughness value related to the value set on the said limiting means.v

' '2. A device as set forth in claim fin which the certain value set onthe limiting means is adjustl f able.

3. In combination, an electrical pickup device of the type whichestablishes a iiuctuating electrical quantity related to excursions of astylus trical quantity related to excursions of' said stylus withrespect to the carrier means, a.v source of constant amplitudeelectrical current. current measuring means, means for setting areference level for said fluctuating electrical quantity,l and means forconnecting the source of constant amplitude 'electrical current to saidcurrent measuring means only during the time that the value of thefluctuating electrical quantity ex.

pickup device of the'type adapted to establish a.

fluctuating electrical quantity related to roughness induced excursionsof a stylus as said stylus and the specimen are moved relative to each.other, the combination of means for amplifying said electrical quantityand having a push-pull output. means for rectifying said electricalquantity, means for setting a reference level for said fluctuatingelectrical quantity, a source of constant amplitude electrical current,means for measuring electrical current, and means for connecting .thesource of constant electrical current to said current measuring meansonly during. the

time that the value' of the iluctuatlng electrical quantity exceeds thevalue of the set reference level.

14. A device as set forth in claim 13, furtherV4 characterized by saidcurrent measuring means being of the integrating type.

15. A device as set forth in claim 13 further characterized by switchmeans in the push-pull output of said ampliiier for selectively connect-'surface roughness peaks or surface roughness valleys or total roughnessexceeding a certain value corresponding to the roughness value oi theset reference level.

BERT A. COSS.

